build: move e2e dependencies into e2e/go.mod

Several packages are only used while running the e2e suite. These
packages are less important to update, as the they can not influence the
final executable that is part of the Ceph-CSI container-image.

By moving these dependencies out of the main Ceph-CSI go.mod, it is
easier to identify if a reported CVE affects Ceph-CSI, or only the
testing (like most of the Kubernetes CVEs).

Signed-off-by: Niels de Vos <ndevos@ibm.com>

e2e/vendor/github.com/gogo/protobuf/CONTRIBUTORS

@@ -0,0 +1,23 @@
+Anton Povarov <anton.povarov@gmail.com>
+Brian Goff <cpuguy83@gmail.com>
+Clayton Coleman <ccoleman@redhat.com>
+Denis Smirnov <denis.smirnov.91@gmail.com>
+DongYun Kang <ceram1000@gmail.com>
+Dwayne Schultz <dschultz@pivotal.io>
+Georg Apitz <gapitz@pivotal.io>
+Gustav Paul <gustav.paul@gmail.com>
+Johan Brandhorst <johan.brandhorst@gmail.com>
+John Shahid <jvshahid@gmail.com>
+John Tuley <john@tuley.org>
+Laurent <laurent@adyoulike.com>
+Patrick Lee <patrick@dropbox.com>
+Peter Edge <peter.edge@gmail.com>
+Roger Johansson <rogeralsing@gmail.com>
+Sam Nguyen <sam.nguyen@sendgrid.com>
+Sergio Arbeo <serabe@gmail.com>
+Stephen J Day <stephen.day@docker.com>
+Tamir Duberstein <tamird@gmail.com>
+Todd Eisenberger <teisenberger@dropbox.com>
+Tormod Erevik Lea <tormodlea@gmail.com>
+Vyacheslav Kim <kane@sendgrid.com>
+Walter Schulze <awalterschulze@gmail.com>